The present invention relates generally to semiconductors and, more particularly, to diodes fabricated in monolithic semiconductor devices.
An ordinary P-N junction diode comprises a semiconductor having an N region suitably doped to provide negative charge carriers and an adjacent P region suitably doped to provide positive charge carriers. The boundary between these regions defines the P-N junction. A depletion zone, within which there are no free charge carriers, extends a short distance into the semiconductor on both sides of the P-N junction. The depletion zone can be considered as a capacitor having a dielectric defined by the semiconductor material enclosed within the zone, and plates defined by the borders between the depletion zone and the adjacent N and P regions. As a result a P-N diode exhibits capacitive as well as diode characteristics.
If a reverse bias voltage is applied across a P-N diode, the depletion zone widens. As the voltage is increased, so does the width of the depletion zone. Widening the depletion zone is effectively the same as physically increasing the separation between the capacitor plates defined by the depletion zone boundaries, and accordingly the capacitance exhibited by the diode varies with the applied voltage. This voltage-variable capacitance has a negligible effect in most applications of diodes. In some other applications it is even used to advantage. However, the capacitive effect is neither negligible nor advantageous in most microwave diode applications, and hence, like other unwanted stray capacitances, it must be compensated for. Since it is considerably easier to compensate for a fixed stray capacitance than for a variable one, a diode having a capacitance that remains constant offers advantages to the microwave circuit designer. A PIN diode exhibits just such a fixed capacitance, and accordingly PIN diodes are now widely used in microwave applications.
In a PIN diode, a lightly doped intrinsic layer, or I region, is sandwiched between the P and N regions. The P-N junction is defined at the boundary between the I region and one of the other regions, and the depletion zone extends from the P-N junction into the I region. When a reverse bias voltage is applied, the depletion zone expands until it fills the entire width of the I region, but it does not expand further, even if the applied voltage continues to increase. Therefore, the capacitance of the diode stabilizes once the depletion zone has fully occupied the I region. The voltage at which the depletion zone reaches its maximum width is called the "punch-through" voltage, because at that voltage an electric field associated with the depletion zone is said to "punch through" the I region to the region beyond.
Although the use of discrete PIN diodes has become widespread, so far it has not been possible to fabricate PIN diodes in monolithic semiconductor devices. In a prior attempt to fabricate a PIN mesa diode in monolithic form, the desirable stabilization of stray capacitance has not been achieved at microwave frequencies, and the principal advantage of the PIN diode has therefore been lost. Accordingly, there is still a need for a reliable technique for producing PIN diodes in monolithic form, especially those suited for use at microwave frequencies. Such devices have application in microwave monolithic control circuits, beam lead PIN diodes, glass isolated series (PIN) switches on a single chip, and frequency multipliers used in surveillance and theft detection systems. The present invention satisfies this need.